1. Field of the Invention
This invention relates to an improved fuel injector for internal combustion engines, and to a method of producing the fuel injector.
2. Description of the Prior Art
For supplying fuel to internal combustion engines, fuel injection valves are often used at present. For meeting emissions limits and reducing fuel consumption, the fuel quantity injected into the individual cylinders must be dimensioned precisely. This requires extremely short opening and closing times of the injection valve. Moreover, the valve needle must have the same stroke each time it opens, to assure that the same quantity of fuel will be injected into the cylinder each time. This is attained by means of a stroke limitation.
In the fuel injectors that are available on the market, the limitation of the stroke of the valve needle is done in various ways. For instance, the stroke limitation is realized by using a stop disk, which is struck by the valve needle. The stop disk rests on the magnet cup, so that a direct impact of the valve needle on the magnet cup is avoided. The stroke required for operation of the fuel injector and the remanent air gap are adjusted by grinding protrusions or steps into the valve needle.
Another possible way of attaining the stroke limitation is for the valve needle to strike a sleeve press-fitted into the magnet cup. In this variant, the stroke and the remanent air gap are adjusted by adjusting disks, whose thickness is adapted to the stroke into the remanent air gap.
Another possible way of attaining the stroke limitation is to use a holding-down device with a sleevelike stop that is surrounded by the magnet cup. One such sleevelike stop is known from German Patent Disclosure DE 102 49 161 B3. In it, the stroke is adjusted by an adjusting disk between the valve housing and the holding-down device, and the remanent air gap is adjusted by grinding the sleeve, embodied as a stop, down to the appropriate length.
To avoid eddy currents in the magnet cup, which slow down the switching of the fuel injector, the magnet cup is made from a metal-polymer composite material, as described in Bosch Research Info, 3/2001. To that end, fine iron particles are sheathed in plastic, compacted, and sintered to make a workpiece. However, this material is very brittle and therefore vulnerable to impacts. For this reason, the valve needle must be prevented from striking the magnet cup.
If the valve needle strikes the magnet cup, parts can break off, causing the magnetic properties of the magnet cup to change. The broken-off particles may also cause increased wear and thus lead to the failure of the fuel injector.